1. Field of the Invention
The invention relates to modulating data for storage within and retrieval from data storage devices such as holographic memory devices and other data storage media.
2. Description of the Related Art
Improving the accuracy of the retrieval of data stored within, e.g., holographic and other page-wise memory devices continues to remain key to successful and widespread implementation of these memory systems. Page-wise memory systems are characterized by the storage and/or readout of an entire page of information in a single operation. That is, unlike conventional memory devices, which store data members in strings or bits, page-wise memories store and retrieve data members in pages in the form of multi-dimensional arrayed patterns of data representations.
A holographic memory or data storage system typically involves three-dimensional storage of holographic representations (i.e., holograms) of data images as a pattern of varying refractive index and/or absorption imprinted in a storage medium such as a crystal of lithium niobate. Holographic memory systems are characterized by their high density storage potential and the potential speed with which the stored data is randomly accessed and transferred.
When stored data is to be recalled from page-wise memory systems including holographic memory systems, detector arrays are used for reading the information emerging from the memory pages. Typically, the recalled holographic information is projected onto an imaging unit such as a charged coupled device (CCD) or a CMOS detector array such as an active pixel sensor (APS), which are sensitive to the emerging optical information. A decoding unit coupled to the imaging unit then determines the corresponding values of the recovered data, which hopefully corresponds to the initially encoded data that was holographically stored.
However, reproduction of data images stored in holographic storage devices are less than ideal due to effects often inherent in holographic memory systems. For example, the number of data members imprinted in the storage medium typically is inversely proportional to the diffraction efficiency and thus the reliability of the holograms stored therein to be reproduced upon read out. Other factors often affecting the quality of the stored data include time-dependent fluctuations in light intensity throughout the storage medium, physical and optical disturbance of the mechanical relationship between memory system components, thermal expansion and other temperature-induced variations throughout the system, and other noise introduced into the system.
Because of the often poor quality in retrieving or reproducing such data, the value of an individual data member often is represented as the comparison of multiple data members or by some similar other encoding technique. For example, it is possible to employ a differential encoding scheme in which a particular data value sought to be retrieved results from the comparison of one retrieved data member relative to another. Alternatively, it is possible to use a reference encoding scheme, in which the value of a data member is compared to one or more known reference data member values. See, for example, U.S. Pat. No. 5,450,218 and U.S. Pat. No. 5,838,469 entitled "Apparatus And Method For Processing Data Stored In Page-Wise Memory", filed on even date herewith.
However, such encoding schemes require the use of more than one data member to represent the data value of a single data member. It is this data "overhead" that reduces data storage efficiency. But, see, for example, co-pending application "Comparator-Based Thresholding Method For Determining Data Values", Ser. No. 08/777,154, Atty. Docket: Campbell 4-12-5, filed on even date herewith, in which data storage overhead is reduced by using actual data members to assist in data value determination.
Still, the inherent effects within such memory systems cause inconsistent reproduction within data pages and from data page to data page. Thus, there exists a need for estimating or otherwise compensating for inconsistent reproduction of data images.